Charles Bolden Jr. knows a thing or two about steering complex hardware through turbulence. He flew combat missions in Vietnam. He piloted two space shuttle missions and commanded two others.

Now, as President Obama's nominee to head the National Aeronautics and Space Administration, the former Marine brigadier general has been tapped to steer the agency through a potentially uncertain transition. The space shuttle program is winding down, and its replacement – the nearly five-year-old Constellation program – is under review. Until that review is complete, budget entries for the program beyond the current fiscal year carry an asterisk.

Pending confirmation, it now falls to Mr. Bolden to guide NASA through the tough decisions ahead on the future of human spaceflight. "What we've got is a situation where we're going to wipe the slate clean, and we're going to do something different," says Roger Launius, a former NASA historian and now curator of the Smithsonian Institution's National Air and Space Museum.

At first glance, any idea of scuttling Constellation might seem quixotic. At the end of this month, NASA program managers are turning over pad 34-B at the Kennedy Space Center to the Constellation team. The program is preparing for the first suborbital flight test of the Ares 1 rocket, currently slated for Aug. 30.

But past administrations have not been shy about pulling the financial plug on previous projects aimed at replacing the space shuttles – notably McDonnell Douglas's Delta Clipper and Lockheed-Martin's X-33/Venture Star, notes Howard McCurdy, a historian at American University in Washington. Both were canceled well into their development phases due to mishaps and technological hurdles that threatened to push up costs.

A commission is currently reviewing NASA's human spaceflight program, and it is not necessarily bound to hold onto Constellation, according to its chairman, Norman Augustine. "We will be looking at different architectures, as well as the existing architecture, and I am not in a position to make any predictions," he said during a briefing earlier this month. "We have been asked to provide options."

One important political factor that could keep Constellation on track: Without it, US astronauts would have to rely on Russian Soyuz launches to reach the International Space Station far longer than they would otherwise would. An approach other than Constellation would take years to develop. It is a potential gap in US capabilities that former NASA administrator Michael Griffin called "unseemly."

The commission's recommendation is due out in August. One of the biggest challenges the new NASA administrator will face is keeping the agency and politicians focused on whatever path the Obama administration chooses afterward.

"There's a lot of frustration within the space community that so many replacements for the space shuttle have been attempted and then not followed through," Dr. McCurdy says.

The administrator's role is a constant test of political and personnel skills. As a presidential appointee, an administrator must sell an administration's agenda to the agency, while standing up for the agency before the president and Congress.

Bolden can do both, say people inside and outside the agency. It's hard to become a general officer in the military without honing political skills, and he counts among his former shuttle crew mates Sen. Bill Nelson (D) of Florida. Senator Nelson served as a mission specialist on a flight in January 1986 and now chairs the Senate Commerce Committee's Space, Aeronautics, and Related Sciences subcommittee. He argued intensively for Bolden as NASA's new administrator.

Moreover, Bolden's long association with NASA, including his service on the agency's Aerospace Safety and Advisory Panel, have reassured people working for the agency.